High-speed quantum memory for photons

Physicists from the University of Basel have developed a memory that can store photons. These quantum particles travel at the speed of light and are thus suitable for high-speed data transfer. The researchers were able to store them in an atomic vapor and read them out again later without altering their quantum mechanical properties too much. This memory technology is simple and fast and it could find application in a future quantum Internet. The journal Physical Review Letters has published the results.

Even today, fast data transfer in telecommunication networks employs short light pulses. Ultra broadband technology uses optical fiber links through which information can be transferred at the speed of light. At the receiver's end, the transmitted information has to be stored quickly and without errors so that it can be processed further electronically on computers. To avoid transmission errors, each bit of information is encoded in relatively strong light pulses that each contain at least several hundreds of photons.

For several years, researchers all over the world have been working on operating such networks with single photons. Encoding one bit per photon is not only very efficient, but it also allows for a radically new form of information processing based on the laws of quantum physics. These laws allow a single photon to encode not only the states 0 or 1 of a classic bit, but also to encode a superposition of both states at the same time. Such quantum bits are the basis for quantum information processing that could make unconditionally secure communication and super fast quantum computers possible in the future. The ability to store and retrieve single photons from a quantum memory is a key element for these technologies, which is intensively investigated.

Simple and fast

A team of physicists led by the professors Philipp Treutlein and Richard Warburton from the University of Basel has now developed a particularly simple and fast quantum memory that stores photons in a gas of rubidium atoms. A laser controls the storage and retrieval processes. The technology used does not require cooling devices or complicated vacuum equipment and can be implemented in a highly compact setup. The researchers were also able to verify that the memory has a very low noise level and is suitable for single photons.

One step closer to the quantum internet

"The combination of a simple setup, high bandwidth and low noise level is very promising for future application in quantum networks," says Janik Wolters, first author of the study. The development of such quantum networks is one of the goals of the National Center of Competence in Quantum Science and Technology (NCCR QSIT) and of the EU Framework Programme for Research and Innovation that have funded this study. In the future, quantum networks could lead to unconditionally secure communication, the networking of different quantum computers and the simulation of complex physical, chemical and biological systems.

Simple Atomic Quantum Memory Suitable for Semiconductor Quantum Dot Single Photons
Janik Wolters, Gianni Buser, Andrew Horsley, Lucas Béguin, Andreas Jöckel, Jan-Philipp Jahn, Richard J. Warburton, and Philipp Treutlein
Phys. Rev. Lett. 119, 060502 – Published 8 August 2017

DOI: https://doi.org/10.1103/PhysRevLett.119.060502

University of Basel

Doubling down on Schrödinger’s cat

Yale physicists have given Schrödinger’s famous cat a second box to play in, and the result may help further the quest for reliable quantum computing.

Schrödinger’s cat is a well-known paradox that applies the concept of superposition in quantum physics to objects encountered in everyday life. The idea is that a cat is placed in a sealed box with a radioactive source and a poison that will be triggered if an atom of the radioactive substance decays. Quantum physics suggests that the cat is both alive and dead (a superposition of states), until someone opens the box and, in doing so, changes the quantum state.

This hypothetical experiment, envisioned by one of the founding fathers of quantum mechanics in 1935, has found vivid analogies in laboratories in recent years. Scientists can now place a wave-packet of light composed of hundreds of particles simultaneously in two distinctly different states. Each state corresponds to an ordinary (classical) form of light abundant in nature.

A team of Yale scientists created a more exotic type of Schrödinger’s cat-like state that has been proposed for experiments for more than 20 years. This cat lives or dies in two boxes at once, which is a marriage of the idea of Schrödinger’s cat and another central concept of quantum physics: entanglement. Entanglement allows a local observation to change the state of a distant object instantaneously. Einstein once called it “spooky action at a distance,” and in this case it allows a cat state to be distributed in different spatial modes.

The Yale team built a device consisting of two, 3D microwave cavities and an additional monitoring port — all connected by a superconducting, artificial atom. The “cat” is made of confined microwave light in both cavities.

“This cat is big and smart. It doesn’t stay in one box because the quantum state is shared between the two cavities and cannot be described separately,” said Chen Wang, a postdoctoral associate at Yale and first author of a study in the journal Science, describing the research. “One can also take an alternative view, where we have two small and simple Schrodinger’s cats, one in each box, that are entangled.”

The research also has potential applications in quantum computation. A quantum computer would be able to solve certain problems much faster than classical computers by exploiting superposition and entanglement. Yet one of the main problems in developing a reliable quantum computer is how to correct for errors without disturbing the information.

“It turns out ‘cat’ states are a very effective approach to storing quantum information redundantly, for implementation of quantum error correction. Generating a cat in two boxes is the first step towards logical operation between two quantum bits in an error-correctible manner,” said co-author Robert Schoelkopf, Sterling Professor of Applied Physics and Physics, and director of the Yale Quantum Institute.

Schoelkopf and his frequent collaborators, Michel Devoret and Steve Girvin, have pioneered the field of circuit quantum electrodynamics (cQED), providing one of the most widely used frameworks for quantum computation research. Devoret, Yale’s F.W. Beinecke Professor of Physics, and Girvin, Yale’s Eugene Higgins Professor of Physics and Applied Physics, are co-authors of the paper.

The research builds upon more than a decade of development in cQED architecture. The Yale team designed a variety of new features, including cylindrical 3D cavities with record quantum information storage time of more than 1 millisecond in superconducting circuits, and a measurement system that monitors certain aspects of a quantum state in a precise, non-destructive way. “We have combined quite a lot of recent technologies here,” Wang said.

Additional co-authors from the Yale Departments of Applied Physics and Physics include assistant professor Liang Jiang; senior research scientist Luigi Frunzio; postdoctoral associates Reinier Heeres and Nissim Ofek; graduate students Yvonne Gao, Philip Reinhold, Kevin Chou, Christopher Axline, Matthew Reagor, Jacob Blumoff, and Katrina Sliwa; and former Yale researcher Mazyar Mirrahimi.

Yale

The quantum Fredkin gate has been experimentally realised for the first time

Researchers from Griffith University and the University of Queensland have overcome one of the key challenges to quantum computing by simplifying a complex quantum logic operation. They demonstrated this by experimentally realising a challenging circuit -- the quantum Fredkin gate -- for the first time.

"The allure of quantum computers is the unparalleled processing power that they provide compared to current technology," said Dr Raj Patel from Griffith's Centre for Quantum Dynamics.

"Much like our everyday computer, the brains of a quantum computer consist of chains of logic gates, although quantum logic gates harness quantum phenomena."

The main stumbling block to actually creating a quantum computer has been in minimising the number of resources needed to efficiently implement processing circuits.

"Similar to building a huge wall out lots of small bricks, large quantum circuits require very many logic gates to function. However, if larger bricks are used the same wall could be built with far fewer bricks," said Dr Patel.

"We demonstrate in our experiment how one can build larger quantum circuits in a more direct way without using small logic gates."

At present, even small and medium scale quantum computer circuits cannot be produced because of the requirement to integrate so many of these gates into the circuits. One example is the Fredkin (controlled- SWAP) gate. This is a gate where two qubits are swapped depending on the value of the third.

Usually the Fredkin gate requires implementing a circuit of five logic operations. The research team used the quantum entanglement of photons -- particles of light -- to implement the controlled-SWAP operation directly.

"There are quantum computing algorithms, such as Shor's algorithm for factorising prime numbers, that require the controlled-SWAP operation.

The quantum Fredkin gate can also be used to perform a direct comparison of two sets of qubits (quantum bits) to determine whether they are the same or not. This is not only useful in computing but is an essential feature of some secure quantum communication protocols where the goal is to verify that two strings, or digital signatures, are the same," said Professor Tim Ralph from the University of Queensland.

Professor Geoff Pryde, from Griffith's Centre for Quantum Dynamics, is the project's chief investigator.

"What is exciting about our scheme is that it is not limited to just controlling whether qubits are swapped, but can be applied to a variety of different operations opening up ways to control larger circuits efficiently," said Professor Pryde.

"This could unleash applications that have so far been out of reach."

The team is part of the Australian Research Council's Centre for Quantum Computation and Communication Technology, an effort to exploit Australia's strong expertise in developing quantum information technologies.

The research has been published as A quantum Fredkin gate in Science Advances(DOI:10.1126/sciadv.1501531)

Griffith University

New quantum computer, based on five atoms

Researchers have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. Image: Jose-Luis Olivares/MIT

The beginning of the end for encryption schemes?

What are the prime factors, or multipliers, for the number 15? Most grade school students know the answer — 3 and 5 — by memory. A larger number, such as 91, may take some pen and paper. An even larger number, say with 232 digits, can (and has) taken scientists two years to factor, using hundreds of classical computers operating in parallel.

Because factoring large numbers is so devilishly hard, this “factoring problem” is the basis for many encryption schemes for protecting credit cards, state secrets, and other confidential data. It’s thought that a single quantum computer may easily crack this problem, by using hundreds of atoms, essentially in parallel, to quickly factor huge numbers.

In 1994, Peter Shor, the Morss Professor of Applied Mathematics at MIT, came up with a quantum algorithm that calculates the prime factors of a large number, vastly more efficiently than a classical computer. However, the algorithm’s success depends on a computer with a large number of quantum bits. While others have attempted to implement Shor’s algorithm in various quantum systems, none have been able to do so with more than a few quantum bits, in a scalable way.

Now, in a paper published today in the journal Science, researchers from MIT and the University of Innsbruck in Austria report that they have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. The system is designed in such a way that more atoms and lasers can be added to build a bigger and faster quantum computer, able to factor much larger numbers. The results, they say, represent the first scalable implementation of Shor’s algorithm.

“We show that Shor’s algorithm, the most complex quantum algorithm known to date, is realizable in a way where, yes, all you have to do is go in the lab, apply more technology, and you should be able to make a bigger quantum computer,” says Isaac Chuang, professor of physics and professor of electrical engineering and computer science at MIT. “It might still cost an enormous amount of money to build — you won’t be building a quantum computer and putting it on your desktop anytime soon — but now it’s much more an engineering effort, and not a basic physics question.”

Seeing through the quantum forest

In classical computing, numbers are represented by either 0s or 1s, and calculations are carried out according to an algorithm’s “instructions,” which manipulate these 0s and 1s to transform an input to an output. In contrast, quantum computing relies on atomic-scale units, or “qubits,” that can be simultaneously 0 and 1 — a state known as a superposition. In this state, a single qubit can essentially carry out two separate streams of calculations in parallel, making computations far more efficient than a classical computer.

In 2001, Chuang, a pioneer in the field of quantum computing, designed a quantum computer based on one molecule that could be held in superposition and manipulated with nuclear magnetic resonance to factor the number 15. The results, which were published in Nature, represented the first experimental realization of Shor’s algorithm. But the system wasn’t scalable; it became more difficult to control the system as more atoms were added.

“Once you had too many atoms, it was like a big forest — it was very hard to control one atom from the next one,” Chuang says. “The difficulty is to implement [the algorithm] in a system that’s sufficiently isolated that it can stay quantum mechanical for long enough that you can actually have a chance to do the whole algorithm.”

“Straightforwardly scalable”

Chuang and his colleagues have now come up with a new, scalable quantum system for factoring numbers efficiently. While it typically takes about 12 qubits to factor the number 15, they found a way to shave the system down to five qubits, each represented by a single atom. Each atom can be held in a superposition of two different energy states simultaneously. The researchers use laser pulses to perform “logic gates,” or components of Shor’s algorithm, on four of the five atoms. The results are then stored, forwarded, extracted, and recycled via the fifth atom, thereby carrying out Shor’s algorithm in parallel, with fewer qubits than is typically required.

The team was able to keep the quantum system stable by holding the atoms in an ion trap, where they removed an electron from each atom, thereby charging it. They then held each atom in place with an electric field.

“That way, we know exactly where that atom is in space,” Chuang explains. “Then we do that with another atom, a few microns away — [a distance] about 100th the width of a human hair. By having a number of these atoms together, they can still interact with each other, because they’re charged. That interaction lets us perform logic gates, which allow us to realize the primitives of the Shor factoring algorithm. The gates we perform can work on any of these kinds of atoms, no matter how large we make the system.”

Chuang’s team first worked out the quantum design in principle. His colleagues at the University of Innsbruck then built an experimental apparatus based on his methodology. They directed the quantum system to factor the number 15 — the smallest number that can meaningfully demonstrate Shor’s algorithm. Without any prior knowledge of the answers, the system returned the correct factors, with a confidence exceeding 99 percent.

“In future generations, we foresee it being straightforwardly scalable, once the apparatus can trap more atoms and more laser beams can control the pulses,” Chuang says. “We see no physical reason why that is not going to be in the cards.”

Mark Ritter, senior manager of physical sciences at IBM, says the group’s method of recycling qubits reduces the resources required in the system by a factor of 3 — a significant though small step towards scaling up quantum computing.

“Improving the state-of-the-art by a factor of 3 is good,” says Ritter. But truly scaling the system “requires orders of magnitude more qubits, and these qubits must be shuttled around advanced traps with many thousands of simultaneous laser control pulses.”

If the team can successfully add more quantum components to the system, Ritter says it will have accomplished a long-unrealized feat.

“Shor's algorithm was the first non-trivial quantum algorithm showing a potential of ‘exponential’ speed-up over classical algorithms,” Ritter says. “It captured the imagination of many researchers who took notice of quantum computing because of its promise of truly remarkable algorithmic acceleration. Therefore, to implement Shor's algorithm is comparable to the ‘Hello, World’ of classical computing.”

What will all this eventually mean for encryption schemes of the future?

“Well, one thing is that if you are a nation state, you probably don’t want to publicly store your secrets using encryption that relies on factoring as a hard-to-invert problem,” Chuang says. “Because when these quantum computers start coming out, you’ll be able to go back and unencrypt all those old secrets.”

MIT

Quantum computer made of standard semiconductor materials

Magnetic field helps qubit electrons store information longer

 

Physicists at the Technical University of Munich, the Los Alamos National Laboratory and Stanford University (USA) have tracked down semiconductor nanostructure mechanisms that can result in the loss of stored information – and halted the amnesia using an external magnetic field. The new nanostructures comprise common semiconductor materials compatible with standard manufacturing processes.

Quantum bits, qubits for short, are the basic logical elements of quantum information processing (QIP) that may represent the future of computer technology. Since they process problems in a quantum-mechanical manner, such quantum computers might one day solve complex problems much more quickly than currently possible, so the hope of researchers.

In principle, there are various possibilities of implementing qubits: photons are an option equally as viable as confined ions or atoms whose states can be altered in a targeted manner using lasers. The key questions regarding their potential use as memory units are how long information can be stored in the system and which mechanisms might lead to a loss of information.

A team of physicists headed by Alexander Bechtold and Professor Jonathan Finley at the Walter Schottky Institute of the Technical University of Munich and the Excellence Cluster Nanosystems Initiative Munich (NIM) have now presented a system comprising a single electron trapped in a semiconductor nanostructure. Here, the electron’s spin serves as the information carrier.

The researchers were able to precisely demonstrate the existence of different data loss mechanisms and also showed that stored information can nonetheless be retained using an external magnetic field.

Electrons trapped in a quantum dot

The TUM physicists evaporated indium gallium arsenide onto a gallium arsenide substrate to form their nanostructure. As a result of the different lattice spacing of the two semiconductor materials strain is produced at the interface between the crystal grids. The system thus forms nanometer-scale “hills” – so-called quantum dots.

When the quantum dots are cooled down to liquid helium temperatures and optically excited, a singe electron can be trapped in each of the quantum dots. The spin states of the electrons can then be used as information stores. Laser pulses can read and alter the states optically from outside. This makes the system ideal as a building block for future quantum computers.

Spin up or spin down correspond to the standard logical information units 0 and 1. But, on top of this come additional intermediate states of quantum mechanical up and down superpositions.

Hitherto unknown memory loss mechanisms

However, there is one problem: “We found out that the strain in the semiconductor material leads to a new and until recently unknown mechanism that results in the loss of quantum information,” says Alexander Bechtold. The strain creates tiny electric fields in the semiconductor that influence the nuclear spin orientation of the atomic nuclei.

“It’s a kind of piezoelectric effect,” says Bechthold. “It results in uncontrolled fluctuations in the nuclear spins.” These can, in turn, modify the spin of the electrons, i.e. the stored information. The information is lost within a few hundred nanoseconds.

In addition, Alexander Bechthold’s team was able to provide concrete evidence for further information loss mechanisms, for example that electron spins are generally influenced by the spins of the surrounding 100,000 atomic nuclei.

Preventing quantum mechanical amnesia

“However, both loss channels can be switched off when a magnetic field of around 1.5 tesla is applied,” says Bechtold. “This corresponds to the magnetic field strength of a strong permanent magnet. It stabilizes the nuclear spins and the encoded information remains intact.”

“Overall, the system is extremely promising,” according to Jonathan Finley, head of the research group. “The semiconductor quantum dots have the advantage that they harmonize perfectly with existing computer technology since they are made of similar semiconductor material.” They could even be equipped with electrical contacts, allowing them to be controlled not only optically using a laser, but also using voltage pulses.

The research was funded by the European Union (S3 Nano and BaCaTeC), the US Department of Energy, the US Army Research Office (ARO), the German Research Foundation DFG (excellence cluster Nanosystems Munich (NIM) and SFB 631), the Alexander von Humboldt Foundation as well as the TUM Institute for Advanced Study (Focus Group Nanophotonics and Quantum Optics).

Publication:

Three-stage decoherence dynamics of an electron spin qubit in an optically active quantum dot; Alexander Bechtold, Dominik Rauch, Fuxiang Li, Tobias Simmet, Per-Lennart Ardelt, Armin Regler, Kai Müller, Nikolai A. Sinitsyn and Jonathan J. Finley; Nature Physics, 11, 1005-1008 (2015) – DOI: 10.1038/nphys3470

Technische Universität München

Tapping Particles of Light

At the Weizmann Institute of Science, researchers have managed to “pluck” a single photon – one particle of light – out of a pulse of light. The findings of this research,which appeared this week in Nature Photonics, bear both fundamental and practical significance: Light is the workhorse of today’s communication systems, and single photons are likely to be the backbone of future quantum communication systems. In addition, say the scientists, the apparatus they have devised will spur further research into the fundamental particle nature of light.  

“Once we move over to quantum communication, information will have to be encoded in single photons,” says Dr. Barak Dayan, head of the Weizmann Institute Quantum Optics group. “Each photon will then represent a single ‘qubit’ – a quantum bit that can exist in more than one state at the same time (for example, an equal combination of both 1 and 0).”

Dayan and his research team, led by Dr. Serge Rosenblum and Orel Bechler, set out to demonstrate a scheme for pulling just one photon out of a stream, on demand. Their mechanism relies on a physical effect that they call single-photon Raman interaction, or SPRINT, which is based on a single atom, or atom-like system. “The advantage of SPRINT,” says Dayan, “is that it is completely passive - it does not require any control fields, just the interaction between the atom and the optical pulse.” In previous research, he and his team had employed SPRINT as a switch for single photons that sent them down different pathways, effectively turning the apparatus into a photonic router. In this work, the atom becomes a tap rather than a switch, snatching one photon from the flow and then turning itself off. “It is not trivial,” says Dayan, “to have a mechanism that continues to function even in high fluxes of photons and to remove just one photon.” 

The experimental setup of Weizmann’s quantum optics group relies on state-of-the-art technologies: laser cooling and trapping of atoms (in this case rubidium), the fabrication of chip-based, ultrahigh-quality glass microspheres, and optical nanofibers. 

“The ability to divert a single photon from a flow could be harnessed for various tasks,” says Dayan, “from creating nonclassical states of light that are useful for basic scientific research, through eavesdropping on imperfect quantum-cryptography systems that rely on single photons, to increasing the security of your own quantum-communication systems. 

The existence of photons was first suggested by Einstein in 1905, yet many of their properties are just now coming to light. Dayan believes their new method will expand our capabilities to study and control them as individual particles.

Weizmann Institute of Science 

Superconductor survives ultra-high magnetic field

Physicists from the universities of Groningen and Nijmegen (the Netherlands) and Hong Kong have discovered that transistors made of ultrathin layers molybdeendisulfide (MoS2) are not only superconducting at low temperatures but also stay superconducting in a high magnetic field. This is a unique phenomenon with exciting promises for the future. The experiments were the first to have been performed at the High Field Magnet Laboratory in Nijmegen, jointly operated by Radboud University and the FOM foundation. The results are published on 12 November by the journal Science.

Superconductivity is a state in which the electrical resistance of a material disappears completely. Normally, this phenomenon only exists at low temperatures and disappears under a high magnetic field. But in the High Field Magnet Laboratory (HFML), physicists discovered that MoS2 -- which can be bought at the home depot stores as dry lubricant -- remains superconducting under a high magnetic field of 37.5 Tesla.

Strongly pinned electron pairs Superconductivity is induced when free electrons in a material are attracted to each other and form weakly bonded electron pairs. These pairs condensate to a superconducting state only when all possible disturbances in the material are minimal and therefore, superconductivity usually exists at very low temperature. When a material is exposed to a magnetic field, the weak bonding between the electron pair can be easily broken. The rupture of the pair destroys the superconductivity when disturbance from the magnetic field becomes strong enough.

Surprisingly, the superconducting state of MoS2 survives high magnetic fields because the paired electrons are intrinsically associated with an internal high magnetic field, which can reach nearly one hundred Tesla, much higher than the 37.5 Tesla provided by the HFML. For comparison: a conventional fridge magnet has a magnetic field of approximately 0.1 Tesla. Uli Zeitler, physicist at the HFML at Radboud University explains: 'MoS2 behaves in a way that contradicts a law in physics, the so called Pauli paramagnetic limit.'

Information in electron spin Although the current publication is very fundamental, Zeitler does have some ideas for future applications. 'There is information stored inside the charge and spin of electrons, the direction of their internal magnetic field. If you can influence this spin, for instance with an electric field, you can store information in there. And in principle, this technique could be used in the development of a future quantum computer.'

The described experiments are complicated to perform. 'We execute this research at very low temperatures between 0 and 12 Kelvin, about minus 270 degrees Celsius, and under high magnetic fields. But in order to pump enough electrons into MoS2, we first needed relatively high temperatures of about minus 50 degrees Celsius', Zeitler explains.

As the first external user of the newly-built 37.5 T magnet, Justin Ye, a physicist from Zernike Institute of Advanced Material at the University of Groningen is really excited by the result: 'It is a good starting point of this new facility. As the first user I am very happy to obtain this important result with the support from my colleagues in HFML. Following this breakthrough in identifying a new paring mechanism with stunning protection of superconductivity against high magnetic field, I would expect a lot of unexpected results from the future experiments in HFML.'

University of Groningen